A so-called binary heat-senstive recording material which utilizes a coloring reaction of a colorless or light-colored dye precursor and a developable compound is disclosed in Japanese Patent Publication Nos. 14039/70 and 4160/68 and Japanese Patent Application (OPI) No. 190886/84 (the term "OPI" as used herein means an "unexamined published application"). Such a binary heat-sensitive recording material has the following practical advantages: (1) it uses primary coloring and thus needs no development; (2) its paper quality is close to that of commonly used paper; (3) it can be easily handled; (4) it can provide a high color density; and (5) it can provide various hues. Therefore, such a binary heat-sensitive recording material is widely used as a heat-sensitive recording material.
In recent years, such a heat-sensitive recording material has been widely used in the printing of bar code labels for sales point data control systems as well as in facsimiles, recorders, and printers.
On the other hand, since such a heat-sensitive recording material utilizes primary coloring, it is disadvantageous in that it undergoes reaction between a dye precursor and a developable compound due not only to application of heat but also upon contact with solvents or the like. This is because such a heat-sensitive recording material comprises organic materials which are highly soluble in solvents and readily react in the solvents. Accordingly, when the heat-sensitive recording material is brought into contact with a stationery containing solvents such as an aqueous or oil ink pen, a fluorescent pen, adhesive, or a plasticizer contained in cooking oil, and wrap film for packaging foods, its background can be colored or its printed portions can be discolored, marring the commercial value.
Efforts have been made in order to eliminate these disadvantages. For example, a protective layer which is resistant to solvents is provided on a heat-sensitive coloring layer as disclosed in Japanese Patent Publication No. 27880/69 and Japanese Patent Application (OPI) Nos. 30437/73 and 31958/73.
In other attempts, two protective layers are provided in order to improve the effectiveness of the various resistances, as disclosed in Japanese Patent Application (OPI) No. 67081/84.
Adhesives such as polyvinyl alcohol and denaturated polyvinyl alcohol have been used as materials for such a protective layer. However, a protective layer provided by laminating two polyvinyl alcohol layers has an insufficient water resistance. Also, a protective layer provided by laminating two layers containing a silicon-containing denaturated polyvinyl alcohol and a colloidal silica has an insufficient resistance to oil. Furthermore, a protective layer provided by laminating a layer of polyvinyl alcohol and a layer of another water-soluble polymer has an insufficient water resistance. Thus, protective layers provided only by laminating unit protective layers have insufficient resistance to various chemicals, water and oil, provide a lower sensitivity, show a poor anti-sticking property, and undergo blocking. Such protective layers also require a complicated production process and thus production costs are relatively high. Therefore, the prior art heat-sensitive recording materials cannot fully satisfy all of the above-noted industrial requirements.